1. Technical Field
The subject invention relates to a driving method of a liquid crystal display device and, more specifically, to a driving method of a liquid crystal display device using an OCB (optically compensated birefringence) technique which realizes a wide view-field angle and high-speed response.
2. Prior Art
Twisted nematic (TN) cells, which at present are widely used in TFT color liquid crystal display devices (TFT/LCDs), have a small view-field angle, which results in a problem that when an LCD panel surface is viewed from an oblique direction, the contrast decreases and image inversion occurs. To solve this problem, i.e., to realize a wide view-field angle, various methods have been proposed. Among those methods is an orientation division method in which each pixel of an LCD is divided into two parts and orientation is effected in different directions in the two parts, and a method in which a pixel electrode is divided into a plurality of parts.
Further, to accommodate a recent trend of multimedia, a motion picture processing function is now necessary which allows a vast amount of image data to be moved on the screen of a liquid crystal display device at high speed. To this end, it is desired that the high-speed response performance of liquid crystal display cells be improved.
In recent years, studies on an OCB cell that is to be used as a liquid crystal cell instead of a TN cell have been made. If the OCB cell technique is used, it becomes possible to obtain a wide view-field angle more easily than with the orientation division method as well as a high-speed response characteristic that is one-order faster than with conventional TN cells. FIG. 13 is a perspective view illustrating the structure of an OCB cell. A liquid crystal material that exhibits bend orientation is sealed between two (top and bottom) glass substrates. Polarizing plates are disposed outside the two respective glass substrates. A phase difference compensation film is disposed between one of the two glass substrates and the polarizing plate.
The bend orientation is orientation in which liquid crystal molecules exist almost only in, for instance, the XZ plane in the coordinate system shown in the figure. In a bend orientation cell, since top and bottom liquid crystal molecules are always oriented symmetrically even if the application voltage is varied, the view-field-angle dependence is symmetrical in the XZ plane. And phase differences in the three-dimensional directions of X, Y and Z are made zero by using the phase difference compensation film (bi-axial film), to obtain a wide view-field angle.
An OCB cell is in a splay orientation state when no bias voltage is applied to it, and exhibits a bend orientation state when a given high voltage is applied to it. To allow an OCB cell to operate as a liquid crystal display device, at a start of operation it needs to be rendered into a bend orientation state from a splay orientation state.
Although they are still in a research stage, there have been started studies on methods of making an OCB cell in a bend state by applying a high voltage between a common electrode and pixel electrodes for a given period at the start of operation of a liquid crystal display device using the OCB cell. However, there occurred unsuccessful cases in which more than several tens of seconds took to attain a bend orientation state, or part of pixels are left untransferred to a bend orientation state. Where an OCB cell does not transfer to a bend orientation state, it is possible for an operator to forcibly attain a bend orientation state by pushing the surface of a region concerned of an LCD panel by his finger. Although this allows the liquid crystal display device to have a desired wide view-field angle characteristic, the image quality required for it cannot be obtained easily. Further, the above measure is not practical.
There is another problem that even if transfer from splay orientation to bend orientation is effected at a start of operation, for a certain reason the OCB cell may return to a splay orientation state during the operation, in which case normal display is not performed unless the power is turned on again.
Recent battery-driven systems such as notebook-type personal computers equipped with a TFT color liquid crystal display device are increasingly required to be of a power-saving type. To attain power-saving, such a liquid crystal display device is provided with a function of stopping it's driving mode to turn off display if, for instance, the system receives no input for a given period. The liquid crystal display device is adapted to instantaneously turn on display upon reception of a signal from the system. However, once drive signals are turned off, an OCB cell returns to a splay orientation state from a bend orientation state. Therefore, certain period is needed to restore the bend orientation state, thus display cannot be turned on instantaneously.